1. Field of the Invention
The present invention relates generally to video signal recording/reproduction devices, and more particularly, to a video signal recording/reproduction device such as a video tape recorder (VTR) for a high definition television comprising dropout compensation function for compensating for dropouts of reproduced video signals.
2. Description of the Background Art
In recent years, the demand for high quality television image is rapidly increasing, whereby the so-called high definition television system having the number of horizontal scanning lines increased significantly in comparison with a conventional television system is being developed to meet these requirements. The development of associated equipments is also in advance to implement the high definition television system. One such equipment is a VTR employing a TCI (Time Compressed Integration) signal as a recording signal format. This TCI signal is obtained by time-base processing the base band signals of a chrominance signal and a luminance signal to time-divisionally multiplex these signals within the same horizontal period. Such VTRs are implemented to FM record the TCI signal obtained by encoding the chrominance signal and the luminance signal in the above described manner on a magnetic tape as a recording medium, at the time of recording, and to decode the TCI signal reproduced from the magnetic tape to restore the chrominance signal and the luminance signal at the time of reproduction.
Missing reproduced video signals in some sections, i.e. dropout of reproduced video signals, due to dust and scratch on the recording medium such as a magnetic tape, often occurs in a video signal recording/reproduction device such as a VTR. Video signal recording/reproduction devices are generally provided with a device compensating for dropout. One such conventional dropout compensation device is disclosed in Japanese Patent Laying-Open No. 63-232778, for example.
FIG. 1 is a block diagram schematically showing a general dropout compensation device adopted in a video signal recording/reproduction device such as a VTR. Referring to FIG. 1, a video signal reproduced from a recording medium such as a magnetic tape (not shown) is converted into a digital data by an A/D converter not shown and provided to a 1H line memory 1 and to one input of an adder 2. A dropout pulse DO, is applied to a 1H memory 3 and an inverter 4 from a dropout detector not shown, which indicates whether there is dropout in the reproduced video signal. This dropout pulse DO, is at a high level when there is no dropout in the reproduced video signal, and at a L level when there is dropout. Dropout pulse DO, is inverted by inverter 4 to be applied to 1H line memory 1 as a write enable signal WE'.
When there is no dropout in the reproduced video signal, signal WE' is at a L level, whereby the reproduced video signal is written into 1H line memory 1. When there is dropout in the reproduced video signal, signal WE' is at the H level, whereby the writing of the reproduced video signal into 1H line memory 1 is inhibited during that period. Regarding the reproduced signal written into 1H line memory 1, the data of the period where dropout occurs is replaced by data of a corresponding period of a reproduced video signal of 1H before held in 1H line memory 1.
The reproduced video signal is delayed by 1H by 1 line memory 1 and applied to one contact 5a of a switch circuit 5 and to the other input of adder 2. Meanwhile, dropout pulse DO, is delayed by 1H by 1H memory 3, and applied to a control input of switch circuit 5 as a selecting signal SEL. During the period where there is no dropout in the reproduced video signal, i.e. the period where signal SEL is at the H level, switch circuit 5 is connected to contact 5a, whereby the reproduced video signal delayed by 1H is directly output from 1H line memory 1 through switch circuit 5. During the period where there is drop out in the reproduced video signal, i.e. the period where signal SEL is at the L level, switch circuit 5 is connected to contact 5b. As a result, the reproduced video signal supplied from 1H line memory 1 and the reproduced video signal currently being written into 1H line memory 1 are added to each other by adder 2, and the amplitude of the resultant signal is attenuated to 1/2 by an attenuator 6. This means that the average value of the signals in the corresponding sections in the reproduced signals of 1H before and 1H after the horizontal period where dropout occurs is output from switch circuit 5.
FIG. 2 is a waveform chart explaining the operation of the dropout compensation device of FIG. 1. Referring to FIG. 2, (a) represents the reproduced video signal applied to 1H line memory 1 and to one input of adder 2. The section marked "S" indicates the dropout. A reference numeral "7" indicates the horizontal synchronizing signal.
The reproduced video signal is delayed by 1H, as shown in FIG. 2 (b), by being stored in 1H line memory 1. The writing of the reproduced video signal into 1H line memory 1 is inhibited during period S where dropout occurs. The data of that section is replaced with data t of the corresponding section T in the reproduced video signal of 1H before.
Then, data t of period S in the reproduced video signal delayed by 1H, and data u of the corresponding section U of the currently reproduced video signal, i.e. the reproduced video signal of 1H after the horizontal period where dropout occurs, are added to each other and averaged according to selecting signal SEL shown in FIG. 2 (c). The dropout of period S is compensated for by the average value v obtained by the above mentioned adding and averaging, as shown in FIG. 2 (d).
A VTR of high definition television system generally carries out recording and reproduction of the TCI signal for each channel by dividing the TCI signal into 2 channels. When dropout is compensated for each channel in such VTRs according to the process shown in FIGS. 1 and 2, an average value of data of 2H before and 2H after the horizontal period where dropout occurs is used for compensation. This means that the correlation between the reproduced video signals is reduced to deteriorate the quality of the picture, in comparison with the case where compensation is carried out using the average value of data of 1H before and 1H after the horizontal period where dropout occurs.
A possible consideration to solve this problem is to compensate for dropout using data of 1H before and 1H after the horizontal period where dropout occurs by substituting data between the two channels of the reproduced TCI signal. However, guard bandless recording of two channels is susceptible to dropout generation in both channels at the same time. Dropout cannot be compensated for sufficiently unless relatively large capacity memories such as frame memories or field memories are used.
There was also a problem that processing circuitry for dropout compensation becomes complicated because the TCI signal with the chrominance signals line sequential processed has different distances upon the time base of data used for dropout compensation regarding the chrominance data section and the luminance data section.